In general, in a joint portion of an arm provided in a construction machine such as a hydraulic excavator, a shaft is inserted into a bearing and repeatedly swings with respect to the bearing in a range of a predetermined turning angle. In this case, the bearing receives high surface pressure of at least 20 MPa. Therefore, as such a bearing, a sliding bearing made of a material with a superior wear resistance is used, and the sliding bearing is used being provided with a high-viscosity lubricant, grease, or wax on a sliding surface thereof. In such a sliding bearing, the sliding surface must be sufficiently supplied with a lubricant so that the sliding bearing smoothly operates without metal contact between the sliding bearing and the shaft and has wear resistance even when the sliding bearing receives a high surface pressure. Accordingly, a material such as a heat-treated carbon steel or a high-strength brass is used as a material for a sliding bearing. Recently, for example, a sintered material disclosed in Japanese Patent Application of Laid-Open No. 2003-222133 may be used as a material for a sliding bearing, and lubricants have been developed. Although the above material and the lubricants have superior characteristics, in order to further extend the life of a bearing, a sliding bearing disclosed in Japanese Patent Application of Laid-Open No. 2006-009846 is also used. In this sliding bearing, plural oblique grooves are formed on an inner circumferential surface so as to extend in a direction crossing the circumferential direction, whereby a lubricant held within the oblique grooves is supplied to a sliding surface as the shaft rotates.
According to the oblique grooves of the bearing disclosed in Japanese Patent Application of Laid-Open No. 2006-009846, the lubricant held within the oblique grooves is drawn by a drawing action generated by the rotation of the shaft. Then, the lubricant flows to the sliding surfaces of lands that are adjacent to the oblique grooves in the rotating direction of the shaft, thereby lubricating the sliding surfaces. In this case, the amount of the lubricant held is increased and is thereby sufficiently supplied to the sliding surfaces. However, there may be cases in which the lubricant is not sufficiently supplied to the sliding surfaces, and wear of the bearing may increase under certain conditions. The inventors have researched the reasons for this and have found the following facts. The lubricant is supplied from the oblique grooves to the sliding surfaces of lands that are adjacent to the oblique grooves, but the lubricant is not sufficient to lubricate the entire area of the sliding surfaces. Therefore, there are some portions in which the lubricant is insufficient, whereby metal contact occurs between the bearing and the shaft at these portions, and wear of the bearing increases.